Liquid crystal electro-optical elements are used for various applications such as not only display devises for office automation equipment, but also measuring devices, automotive instruments, display devices for home electric appliance, clocks and desk-top calculators.
A liquid crystal electro-optical element has such a structure that a pair of substrates, at least one of the substrates having a transparent electrode, an intermediate protective film and a liquid crystal alignment film formed on its surface, is disposed with a certain distance, and a liquid crystal material is enclosed between the substrates, and it functions as an optical switching element by applying a voltage from the electrode to the liquid crystal material to change the alignment state of the liquid crystal material thereby to change its optical properties.
For twisted nematic (TN) and super twisted nematic (STN) liquid crystal electro-optical elements, a liquid crystal composition having a small amount of an optically active compound (chiral agent) added thereto is employed to achieve uniform twist alignment.
Optically active compounds widely used at present may, for example, be a compound of the following formula (CN), a compound of the following formula (CB-15) and a compound of the following formula (R-811). Further, a liquid crystal composition having an optically active compound added thereto is employed also for a reflective cholesteric (chiral nematic) liquid crystal element.

However, a cholesteric liquid crystal (chiral nematic liquid crystal) composition has a high viscosity, and thus it has such problems as a low speed of response and a high driving voltage, since a large amount of the optically active compound is added thereto. There is a corelation between the driving voltage of a liquid crystal element and the dielectric anisotropy (Δ∈) of a liquid crystal composition, and a liquid crystal composition having a high dielectric anisotropy can be driven at a low driving voltage. In order to make a reflective cholesteric (chiral nematic) liquid crystal element be driven at a low voltage, an optically active compound having a high dielectric anisotropy as well as a nematic liquid crystal composition having a high dielectric anisotropy has been required.
The applicant has proposed an optically active compound having a low viscosity and a high helical twisting power in JP-A-11-255675. However, the helical twisting power of the optically active compounds as specifically disclosed in JP-A-11-255675 is insufficient particularly for a reflective cholesteric (chiral nematic) liquid crystal element, and thus a higher helical twisting power is required, and further, optically active compounds having a high dielectric anisotropy which have not been realized in JP-A-11-255675 are required.
The helical pitch length induced when an optically active compound is added to a liquid crystal composition is determined by the helical twisting power characteristic to the compound. The smaller the helical twisting power of an optically active compound, the longer the helical pitch length induced, and the addition amount has to be increased to shorten the helical pitch length. In general, if the addition amount of the optically active compound is increased, performances as a liquid crystal material tend to decrease, as compared with before addition and problems are caused such as an increase in the viscosity, a decrease in the speed of response, an increase in the driving voltage, a decrease in the isotropic phase transition temperature and a reduction of the temperature range at which a specific phase such as a nematic phase or a cholesteric phase is shown. Thus, an optically active compound having a high helical twisting power has been required.
In order to overcome such objects, JP-A-10-251185 proposes optically active compounds having a low viscosity and a high helical twisting power.
In recent years, attention has been paid to a reflective cholesteric liquid crystal element employing a cholesteric liquid crystal composition having a large amount (at a level of from 10 to 30 wt %) of an optically active compound added to a nematic liquid crystal composition, and utilizing such a phenomenon that the cholesteric liquid crystal selectively reflects light having a wavelength equal to the product of the average reflective index of the liquid crystal material and the helical pitch length. The reflective cholesteric liquid crystal element has a high efficiency of light since no deflecting plate or color filter is required, and a higher brightness can be obtained when a liquid crystal composition having a higher reflective index anisotropy (Δn) is employed. Further, a voltage has to be applied only when the changeover of display (writing) since the display state is maintained (memory properties), whereby the element consumes only a low electric power. However, since a large amount of an optically active compound is added, the cholesteric liquid crystal composition has a high viscosity, whereby there are problems such as a low speed of response and a high driving voltage. In order to overcome such problems, an optically active compound which has a helical twisting power higher than that of the optically active compounds as disclosed in JP-A-10-251185, and which provides an aimed helical pitch length even by addition in a small amount, has been required.
Further, many of practicable liquid crystal compositions are prepared by mixing at least one compound having a nematic phase in the vicinity of room temperature and at least one compound having a nematic phase in a temperature region higher than room temperature. Further, in recent years, along with diversification of products to which liquid crystal electro-optical elements are applied, a liquid crystal composition having an increased operating temperature range to the high temperature side has been required, and accordingly, particularly a liquid crystal composition having a high transition temperature from the liquid crystal phase to the isotropic phase [hereinafter referred to as clear point (Tc)] has been required.
Accordingly, an optically active compound which, when added to a liquid crystal composition, does not lower the clear point (Tc) of the liquid crystal composition has been required.
It is an object of the present invention to provide a liquid crystal composition which contains a novel optically active compound having a high helical twisting power, having a high dielectric anisotropy (Δ∈) and providing excellent performances also as a chiral agent, and which can be driven at a low voltage, and a liquid crystal element employing the liquid crystal composition, particularly a STN or cholesteric (chiral nematic) liquid crystal element.